pidns: Deny strange cases when creating pid namespaces.
task_active_pid_ns(current) != current->ns_proxy->pid_ns will
soon be allowed to support unshare and setns.
The definition of creating a child pid namespace when
task_active_pid_ns(current) != current->ns_proxy->pid_ns could be that
we create a child pid namespace of current->ns_proxy->pid_ns. However
that leads to strange cases like trying to have a single process be
init in multiple pid namespaces, which is racy and hard to think
about.
The definition of creating a child pid namespace when
task_active_pid_ns(current) != current->ns_proxy->pid_ns could be that
we create a child pid namespace of task_active_pid_ns(current). While
that seems less racy it does not provide any utility.
Therefore define the semantics of creating a child pid namespace when
task_active_pid_ns(current) != current->ns_proxy->pid_ns to be that the
pid namespace creation fails. That is easy to implement and easy
to think about.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 3cc29b8..0dbbc66b 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -146,6 +146,8 @@
return get_pid_ns(old_ns);
if (flags & (CLONE_THREAD|CLONE_PARENT))
return ERR_PTR(-EINVAL);
+ if (task_active_pid_ns(current) != old_ns)
+ return ERR_PTR(-EINVAL);
return create_pid_namespace(user_ns, old_ns);
}